In optical transmission devices that transmit optical transmission signals, for example, nonlinear distortion due to the nonlinear characteristics of optical fibers or optical transmission lines for amplifiers or the like disposed on optical fibers triggers degradation of the characteristics of optical transmission signals. Thus, in recent years, studies have been conducted on a technology that compensates the nonlinear characteristics of optical transmission lines by using digital signal processing.
FIG. 15 is a block diagram illustrating an example of a reception side digital signal processor (DSP) 100 in an optical receiver in an optical transmission device. The reception side DSP 100 is, for example, a digital signal processing circuit on the reception side embedded in an optical transmission device. The reception side DSP 100 includes a compensating unit 101, an adaptive equalizer (AEQ) 102, a frequency offset compensation unit (FOC) 103, and a carrier phase recovery unit (CPR) 104. The compensating unit 101 performs a compensating process of digital back propagation on reception signals. The compensating unit 101 includes a chromatic dispersion compensation unit (CDC) 101A and a nonlinear compensation unit (NLC) 101B.
The CDC 101A is, for example, a wavelength dispersion compensating unit that compensates wavelength dispersion of optical transmission signals generated in optical fibers. The NLC 101B is, for example, a nonlinear compensating unit that calculates nonlinear distortion generated in the amplitude of a reception signal in an optical fiber and that compensates the nonlinear distortion by subtracting the calculated nonlinear distortion from the reception signal.
The AEQ 102 is an adaptive equalizer that performs polarization division adaptively following a phenomenon, such as polarization fluctuation of a reception signal, time fluctuation of polarization mode dispersion, or the like. Furthermore, the AEQ 102 compensates residual dispersion of wavelength dispersion that is not able to be compensated by the CDC 101A and compensates a narrowed signal band generated in an electricity device, an optical device, or the like.
The FOC 103 is a frequency offset compensating unit that estimates a frequency offset that is a difference between the wavelength frequency of a light source on the transmission side and the wavelength frequency of a light source on the reception side and that compensates the frequency offset. The CPR 104 is, for example, a carrier phase restoring unit that compensates phase noise of the light source or a variation component of a high-speed frequency offset that was not able to be compensated by the FOC 103.
The reception side DSP 100 performs electric conversion on the reception signal received from the optical fiber and inputs the reception signal to the CDC 101A. The CDC 101A compensates the wavelength dispersion inside the reception signal and input the compensated reception signal to the NLC 101B. The NLC 101B compensates nonlinear distortion of the reception signal and inputs the compensated reception signal to the AEQ 102. Furthermore, the compensating unit 101 uses digital back propagation and alternately repeats, in order to take into consideration a variation in waveform due to wavelength dispersion in the optical fiber as nonlinear compensation, wavelength dispersion compensation and nonlinear compensation until the nonlinear distortion is compensated.
The AEQ 102 performs polarization division on the reception signal, compensates residual dispersion of the reception signal or the narrowed signal band and inputs the compensated reception signal to the FOC 103. The FOC 103 estimates a frequency offset, compensates the estimated frequency offset, and inputs the compensated reception signal to the CPR 104. The CPR 104 compensates phase noise of the light source or a variation component of a frequency offset and outputs the compensated reception signal to a signal processing unit that is not illustrated.
The reception side DSP 100 alternately repeats, by using the compensating unit 101, wavelength dispersion compensation and nonlinear compensation until the nonlinear distortion is compensated and thus the reception side DSP 100 can compensate wavelength dispersion and nonlinear distortion of the reception signal.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-75097
However, in the compensating unit 101 in the optical transmission device, if the signal band width of an optical transmission signal is increased, an amount of the wavelength dispersion to be considered for the nonlinear compensation is increased, which results in an increase in an number of processing steps in which wavelength dispersion compensation and nonlinear compensation are alternately repeated by digital back propagation. Consequently, in the compensating unit 101, as the number of processing steps is increased, calculation of the nonlinear characteristics is more complicated and thus the size of the processing circuit is increased.